Bone cements have been used for a number of years in order to assist the attachment of artificial implants to the skeleton. The cement which serves as a junction between the bone and the implant must meet a certain number of requirements, in particular mechanical, but must also be non-toxic and biocompatible. Certain cements have even been studied for their bioactive properties, i.e. for their action which assists the adhesion and the cellular growth on the implant.
In the United States, between 400,000 and 500,000 clinical osteoporotic vertebral fractures occur annually. Approximately one third of these patients develop chronic, debilitating pain that does not respond to conservative treatment. For many people, this represents the end of their independent lifestyle, sometimes leading to a worse than expected overall survival rate.
Many of these patients can be effectively treated by the percutaneous injection of bone cement into the fractured vertebral body. This procedure results in significant reduction in pain in approximately 80% of patients treated by vertebroplasty, in addition to strengthening of the diseased bone.
Since the mid 1980's, the use of cements has widened to bone repair, and primarily to percutaneous vertebroplasty. This minimally invasive technique allows injection of a cement through a trocar into a fractured vertebra in order to ensure bone volume and stabilization. The first percutaneous vertebroplasty was achieved in 1984 and has since enjoyed increasing success, opening the way to plastic repair of other types of bone.
Vertebroplasty is a minimally invasive surgical technique that has been introduced to medically manage vertebral compression fractures. In this procedure, bone cement is injected percutaneously, through a long cannula and into the vertebral cancellous bone. Once hardened, the cement reinforces mechanically the weakened vertebra. The principal benefit of this procedure is that up to 90% of the patients experience pain relief within 24 Hours. (Jensen, M. E. et al. (1997). Am J Neuroradiol 18: 1897-1904.) The procedure aims to augment the weakened vertebral body and stabilize it. A dough of an injectable bone cement is carefully injected directly into the fractured vertebral body.
Even if this procedure is gaining acceptance, there is still concern regarding accompanying risks. In particular, extraosseous cement leakage, which occurs in up to 73% of the procedures can cause significant side effects. (Barragan-Campos, H. M., et al. (2006). Radiology 238(1): 354-62). This is currently one of the most serious complications. Leaks occur through a path caused by irregularities in the structure, such as blood vessels and/or damage to the vertebral wall. Ideally, the cement should be of moderate viscosity in order to uniformly infiltrate the trabecular bone skeleton, thereby flow paths caused by irregularities.
The cements which have been used to date are organic polymers, formed from a mixture of a prepolymer, generally PMMA (methyl polymethylmethacrylate) and from a monomer, generally MMA (methyl methylmethacrylate), reacting in the presence of a polymerization activator. For use in vivo, which does not allow high temperatures, a reaction initiator is added.
Most commercially available cements are available in the form of two separate components: a powder comprising principally prepolymer balls and a liquid containing principally the monomer. The initiator, for example benzoyl peroxide (BPO), is generally incorporated with the powder, while the liquid contains a chemical activator (catalyst), such as dimethylparatoluidine (DMPT), the polymerization reaction starting when the two components are mixed. In order to avoid spontaneous polymerization which can possibly occur during storage, there is incorporated furthermore in the liquid component a stabilizer, commonly hydroquinone. The activator and the initiator are introduced in the proportion of 1 to 2% in the corresponding component, the stabilizer itself taking effect at some tens of ppm.
In order to display the cement visually during and after the operation by radiological means, a radio-opaque substance can be added, most often barium sulfate (BaSO4) or zirconium dioxide (ZrO2). Commercial cements contain a quantity thereof of the order of 10% in the powder.
These binary compositions for the preparation of bone cements, designed originally for attachment of implants and sealing of prostheses, fulfill the criteria of resistance to traction and to compression, of chemical neutrality and of biocompatibility. They are authorized for medical use and have proved their qualities in the long term when the skeleton is subjected to large, repeated forces. This is why bone cements for attaching implants have been adopted as the preferred material for bone reconstruction surgery.
However, the conditions for using cements in percutaneous surgery requires different properties to prevent accidents, the effects of which can be dramatic for the patient, such as paraplegia. The practitioner must use a cement which is sufficiently fluid for it to flow through a trocar with a diameter of a few millimeters, and for it to maintain this fluidity long enough for the practitioner to have the time to operate at leisure. At the same time, the physical characteristics of the cement after polymerization must be preserved.
Further, the cement injected even in small quantities must be visually displayed during the operation. The currently marketed compositions do not have this property. Although certain cements contain a radio-opacifier, correct visual display during percutaneous injection is not obtained. This leads the practitioners to modify the compositions themselves, with the risk of modifying, at the same time, the physical characteristics of the cement, in particular its viscosity and its rate of hardening, and its resistance properties after polymerization.
Hence, the currently known cements, even if they are efficient in attaching implants to the skeleton, have not been designed for a treatment stabilizing the vertebral body by a percutaneous route and do not take into account the specificities connected to this technique.